Incompatible changes:

The C-ENUM: word in the C library interface has been replaced with ENUM:, a much improved word for defining type-safe enumerations. (Erik Charlebois, Joe Groff)

Tuple slot setter words with stack effect ( value object -- ) are now named foo<< instead of (>>foo). Most code is unaffected since it uses the >>foo form. (Slava Pestov)

The older system-micros word, which returned microseconds since the Unix epoch as an integer, has been removed. For a while, the recommended way to get the current time has been to call the now word from the calendar vocabulary, which returned a timestamp instance. (Doug Coleman)

The alarms vocabulary has been renamed to timers to better explain its true purpose, with improved timing accuracy and robustness. (Doug Coleman)

cocoa.subclassing: the syntax for defining new Objective-C classes has been changed to improve readability. (Slava Pestov)

io.streams.limited: the ability to throw errors on EOF was extracted from limited streams, and limited streams simplified as a result. Throwing on EOF is now implemented by the io.streams.throwing vocabulary. (Doug Coleman)

The code generation for the C library interface has been revamped; details in a blog post. (Slava Pestov)

Something similar to what C++ and C# programmers refer to as "value types"; binary data can now be allocated on the call stack, and passed to C functions like any other pointer. The with-out-parameters combinator replaces tricky code for allocating and juggling multiple temporary byte arrays used as out parameters for C function calls, making this idiom easier to read and more efficient. The with-scoped-allocation combinator presents a more general, lower-level interface. (Slava Pestov)

The compiler can now use the x87 floating point unit on older CPUs where SSE2 is not available. However, this is not recommended, because the build farm does not test Factor (or build any binaries) in x87 mode, so this support can break at any time. To use x87 code generation, you must download the source code and bootstrap Factor yourself, on a CPU without SSE2. (Slava Pestov)

Miscellaneous improvements:

fuel: Factor's Ultimate Emacs Library has seen many improvements, and also some keyboard shortcuts have changed; see the README. (Erik Charlebois, Dmitry Shubin, Jose A. Ortega Ruiz)

A new factor.cmd script is now included in the build-support directory, to automate the update/build/bootstrap cycle for those who build from source. Its functionality is a subset of the factor.sh script for Unix. (Joe Groff)

The default set of icons shipped in misc has been tweaked, with better contrast and improved appearance when scaled down. (Joe Groff)